There is a general need for an integrated circuit (IC) for driving laser-diodes or LEDs in high data rate transmission systems. For these and other applications using LEDs, current driver circuits are needed which may operate at a low headroom voltage while still achieving high drive performance in terms of edge speed and jitter.
VCSEL (Vertical Cavity Surface Emitting Laser) diodes are often used as LEDs. The circular beam of a VCSEL can easily be coupled with a fiber. This is mainly due to the characteristic of VCSEL diodes as surface emission rather than edge emission devices and they are known for their excellent power efficiency and durability. Accordingly, VCSEL diodes are widely used in low cost optical transmission systems. However, in high data rate transmission systems, the VCSEL diodes are turned on and off with very high switching frequencies. This requires fast, power efficient and precise driving stages.
Furthermore, the forward voltage of laser-diodes is a function of the wavelength of the emitted light. It is well known in the art that gallium aresenide or GaAs is used as substrate for 850 nm laser-diodes. The corresponding bandgap energy is approximately 1.5 eV. Other diodes such as, for example, 1310 nm or 1550 nm laser-diodes, use indium phosphide or InP as a substrate.
In these devices the bandgap energy is approximately 1 eV and 0.8 eV, respectively. The forward voltage of a Laser-diode at a temperature of zero Kelvin is equal to the bandgap energy divided by the elementary charge e. Therefore the value of the forward voltage is approximately 1.5V at 850 nm, approximately 1.0V at 1310 nm, and approximately 0.8V at 1550 nm. At higher temperatures the forward voltage also becomes a function of the doping profile of a diode. Depending on the magnitude of the current flowing through the laser-diode and its internal resistance, the voltage drop can reach values of up to 2.1V, e.g., for VCSEL type laser-diodes emitting light at 850 nm wavelength. Since in many systems a minimum supply voltage of 2.9V is used, the remaining headroom for the laser-diode current driving stage can be as low as 0.8V or even less.